1. Field of the Invention
This invention relates generally to filters for use in subterranean environments, such as in oil and gas wells, water wells, and geothermal wells, and in other environments in which it is desired to remove a liquid or gas from the ground without bringing soil particulates, such as sand or clay, up with the liquid or gas.
2. Description of the Related Art
There is much demand for well filters capable of being used in subterranean wells in order to remove particulates from liquids or gases produced by the wells. Typical particulates which need to be filtered out are sand and clay, and for this reason, such filters are often referred to as sand screens.
A well filter typically includes an inner support member, such as a perforated core, and a filter body including a filter medium disposed around the inner support member. In many cases, the well filter will further include an outer protective member, such as a perforated cage, disposed around the filter body for protecting it from abrasion and impacts. In order to facilitate installation of the cage over the filter body, there is usually a clearance between the outer periphery of the filter body and the inner periphery of the outer protective member.
During use of such a well filter in a well, fluid will usually flow radially inwards through the filter body during filtration, so the filter body will be subjected primarily to radially inward forces. However, on occasions, such as during acidizing of a well, injection of mud into a well, air sparging, or momentary pressure reversals (either intentional or accidental), fluid may flow radially outwards through the filter body, producing hoop stresses which stretch the filter medium in the filter body outwards towards the outer protective member. At such times, the pressure differential between the inside and the outside of the filter body during reverse flow may be extremely high, such as on the order of several thousand said greater on the inside than the outside. Because of the clearance between the outer periphery of the filter body and the inner periphery of the cage, there is a likelihood of the hoop stresses exceeding the strength of some portion of the filter body and producing excessive plastic elongation of the filter medium or damage to seams or joints of the filter body. Upon resumption of forward (outside-in) flow through the filter, the filter body may no longer be able to properly perform filtration because particles which should be removed by the filter medium are able to pass through regions which were damaged by the elongation of the filter medium or other portions of the filter body. Even if the filter body is not damaged by a single occurrence of radial expansion, if expansion takes place repeatedly, the filter body may suffer fatigue damage over time.
Accordingly, the present invention provides a filter for subterranean use which is capable of being subjected to radially outward forces without damage to a filter medium of the filter.
The present invention further provides methods of forming such a filter.
According to one form of the present invention, a filter includes a hollow filter body including a filter medium, and a restraining member disposed around the filter body.
The restraining member may restrain the filter body against radial expansion caused by radially outward forces encountered during operation of the filter within a well and prevent the filter body from exceeding its maximum allowable elongation, whereby the filter body can be prevented from being damaged by radial expansion. The filter may also include an inner support member surrounded by the filter body and capable of transporting a fluid in its axial direction.
The inner periphery of the restraining member may be in contact with the outer periphery of the filter body when no external radial forces are acting on the filter, or it may be spaced from the outer periphery of the filter body by a gap. Any gap is preferably sufficiently small that the filter body can expand against the inner surface of the restraining member without exceeding the maximum allowable elongation of the filter body. The maximum allowable elongation is the maximum elongation (expressed as strain) at which the filter body can still remove particles of a given size with a desired efficiency.
The restraining member may have a variety of forms. For example, it may be a perforated cage or a wrap member which is helically wrapped around the filter body. The wrap member may define openings through which particles at least as large as 0.015 inch in diameter can pass. The wrap member may be wrapped around the filter body in several turns and may radially compress the filter body. The wrap member may have a non-circular cross section and may be twisted along its longitudinal axis.
According to another form of the present invention, a filter includes a hollow filter body and a perforated cage surrounding the filter body. The filter body includes a metallic filter medium capable of resisting tension in a circumferential direction. An outer periphery of the filter body is able to contact an inner periphery of the cage around an entire circumference of the filter body without exceeding a maximum allowable elongation of the filter body. An inner diameter of the cage increases towards lengthwise ends of the cage.
According to still another form of the present invention, a method of forming a filter includes disposing a perforated cage around a filter body including a filter medium, and plastically deforming the cage radially inwards to reduce an inner diameter of the cage. For example, the inner diameter of the cage may be reduced to a level such that the filter body can contact the inner periphery of the cage without exceeding the maximum allowable elongation of the filter body.
A filter according to the present invention can be employed in any desired manner in a wide variety of underground environments, such as in oil or gas wells, water wells, geothermal wells, groundwater remediation wells, and leaching ponds. For example, the filter can be used in cased hole gravel pack completion in which the filter is disposed inside a gravel pack in the production zone of an oil or gas well, in an open hole gravel pack in which the filter is disposed in a gravel pack without being surrounded by casing, in filtration without a gravel pack, in open hole completion in which the filter is introduced into a well as part of a well string and the formation surrounding the well bore is allowed to fall in on the filter, in slim hole completion in which the filter is installed in a drill string and left in the well with the drill string at the completion of drilling, in coiled tubing completions and workovers in which the filter is connected to coiled tubing, with wire-line in which the filter is lowered into a well by wire-line and sealed in place in the well, for pump protection in which the filter is installed on or upstream of the inlet of a subsurface pump, in ground remediation in which underground fluids are brought to the surface via the well or compressed air is passed through the filter to form air bubbles and perform air sparging, and in leaching ponds for filtering ion-containing liquids which leach from ores in the pond. Details of the manner of using the filter in these and other environments are described in U.S. Pat. No. 5,664,628, which is incorporated by reference.